Vertical distribution of 236U in the North Pacific Ocean

The first extensive study on ²³⁶U in the North Pacific Ocean has been conducted. The vertical distribution of ²³⁶U/²³⁸U isotopic ratios and the ²³⁶U concentrations were analysed on seven depth profiles, and large variations with depth were found. The range of ²³⁶U/²³⁸U isotopic ratios was from (0.09 ± 0.03) × 10^-10 to (14.1 ± 2.2) × 10^-10, which corresponds to ²³⁶U concentrations of (0.69 ± 0.24) × 10⁵ atoms/kg and (119 ± 21) × 10⁵ atoms/kg, respectively. The variations in ²³⁶U concentrations could mainly be attributed to the different water masses in the North Pacific Ocean and their formation processes. Uranium-236 inventories on the water column of each sampling station were calculated and varied between (3.89 ± 0.08) × 10¹² atoms/m² and (7.03 ± 0.50) × 10¹² atoms/m², which is lower than in former studies on comparable latitudes in the North Atlantic Ocean and the Sea of Japan. The low inventories of ²³⁶U found for the North Pacific Ocean in this study can be explained by the lack of additional input sources of artificial radionuclides, apart from global and regional/local fallout. This study expands the use of ²³⁶U as oceanographic circulation tracer to yet another ocean basin and shows that this isotope can be used for tracing circulation patterns of water masses in the Pacific Ocean.

First study on 236U in the Northeast Pacific Ocean using a new target preparation procedure for AMS measurements

We succeeded in obtaining the depth profile of ²³⁶U for a sampling station in the Northeast Pacific Ocean using only one litre of seawater sample from each depth. For this purpose, a new procedure was developed that allowed for the preparation of accelerator mass spectrometry targets for trace uranium using only 100 μg of iron carrier material. The ²³⁶U concentrations in water samples from the Northeast Pacific Ocean showed large variations from (9.26 ± 0.42) × 10⁶ atoms/kg at 60 m depth to (0.08 ± 0.02) × 10⁶ atoms/kg at a depth of 3000 m. The high ²³⁶U concentrations in surface water reflect the input of ²³⁶U by global and local fallout from nuclear weapons tests. The low ²³⁶U concentrations in seawater from 1500 m and below are an indicator for the low vertical diffusion of surface water to deeper layers in the North Pacific Ocean. The total inventory of ²³⁶U on the water column was (8.35 ± 0.23) × 10¹² atoms/m², which is lower compared to those of other ocean regions solely affected by global fallout on comparable latitudes. This study represents the first dataset for ²³⁶U in the Pacific Ocean and shows the possibility of downsizing sample volumes which may help in future applications of ²³⁶U as tracer for large ocean areas.

236U/238U and 240Pu/239Pu isotopic ratios in small (2 L) sea and river water samples

Accelerator Mass Spectrometry (AMS) and alpha spectrometry were used to determine uranium ((236)U, (238)U, (234)U) and plutonium isotopes ((239)Pu, (240)Pu) in sea and river water samples. Plutonium was separated by Dowex(®) 1 × 8 resin and UTEVA(®) resin was used for uranium purification. The measured (236)U/(238)U isotopic ratios for surface water from the Atlantic Ocean, the Pacific Ocean and the Black Sea were in the order of 10(-9), while values for river water were in the order of 10(-8). These contaminations may be attributed to global fallout. A sample of the reference material IAEA-443, collected from the Irish Sea, showed, in accordance to the reference value, a ratio that was 10(3) times higher due to effluents from the reprocessing plant at Sellafield. These results underline the good suitability of (236)U/(238)U as a tracer for hydrology and oceanography, and show that relatively small water samples are sufficient for the determination of (236)U by AMS, which is not the case for plutonium with present techniques. The plutonium concentrations in our water samples could only be measured with large uncertainties and were in the order of 10(-3) mBq/L (with the exception of the Irish Sea sample).